316 



PRINCIPLES OF ANIMAL BIOLOGY 



The interpretations to be put upon the differences in the animals of 

 successive periods of time are manifold. The same group of facts may 

 often be made to bear upon a variety of questions. 



Change of Environment. — Probably nothing will appeal more forcibly 

 to a person who knows something of living animals and the habitats in 

 which they are found, than to discover in his excursions afield the fossil 

 of an animal in a region where it could not possibly exist under present 

 conditions. In Michigan, for example, some of the finest of extinct corals 

 are found (Figs. 212 and 216). Corals are today exclusively marine 



animals, and presumably were marine in 

 earlier times. Yet Michigan is now in 

 mid-continent, hundreds of miles from 

 the nearest salt water. The environment 

 has changed. This is not an isolated 

 instance, it is but one of many similar 

 cases. Continents and shore lines have 

 changed often and greatly, as the fossils of 

 land and marine animals testify. 



The same spot on the earth's surface 

 has been repeatedly dry land and under 

 water. Fossils indicate that swamps have 

 changed to plains, forests to grass-lands 

 and finally to deserts. Climatic changes 

 have been frequent. Tropical tempera- 

 tures, as indicated by luxuriant plant 

 growth and attendant animals, changed 

 to winter 

 ameliorated, 

 arid 



severity. 

 Humid 



w 



hich 



Fig. 216.- — Fossil of cup coral 

 found in Michigan, demonstrating 

 that Michigan was once covered 

 with marine waters. {From speci- 

 men in the Musexim of Geology, 

 University of Michigan.) 



come 



was later 

 regions have be- 

 in many places. No other 

 assumptions will explain the fossils found 

 at different levels in the same region. 



Migrations of Animals. — The changes 

 in the nature of the earth's surface in the 

 past necessitated the migration or extinction of most if not all of 

 the animals inhabiting the changing regions. Some of these earth 

 changes were radical. As Michigan, for example, changed from sea- 

 bottom to dry land by the elevation of the earth's crust, marine animals 

 necessarily either followed the sea as it receded, or became modified so as 

 to be capable of life under new conditions. Probably no animals emerged 

 from the sea, during these changes, to live on land, for alterations to 

 suit land conditions would have involved in most cases both structural 

 and physiological changes, some of them fundamental in nature. Possi- 

 Ijly a few could endure ])rackish, or eventually even fresh water, and 

 remained in streams or inlets, but the majority almost certainly could not 



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